In recent years, efforts have been made to speed up radio communications systems such as microwave cellular phone systems, radio LAN (Local Area Network) systems, etc. by using multilevel modulation/demodulation and multiple carriers. However, there is a limitation imposed on the speeding-up efforts due to a narrow frequency band that can be used. For example, if multilevel PSK (Phase Shift Keying) is employed, then the error rate is degraded and a very high performance level is required for phase noise and frequency stability of the oscillator. If multiple carriers are achieved by OFDM (Orthogonal Frequency Division Multiplexing), then the frequency band is determined by the product of the number of subcarriers and the symbol rate. Therefore, the frequency band needs to be wider as the system Is speeded up. There is known another problem in that since the difference between the peak power and the average power is large, low-distortion transmission amplifiers are generally required.
Consequently, there has been developed a radio communications system which incorporates the MIMO (Multi-Input Multi-Output) technology in the microwave band. FIG. 1 is a block diagram of a radio communications device employing the above technology. Transmitter 500 has transmitting circuits 501-1 through 501-3, antennas 502-1 through 502-3, and transmission signal processing circuit 504. Transmission data is processed by transmission signal processing circuit 504, and radiated as radio waves from antennas 502-1 through 502-3 by transmitting circuits 501-1 through 501-3. Receiver 506 has antennas 508-1 through 508-3, receiving circuits 507-1 through 507-3, and reception signal processing circuit 510. The radio waves received by antennas 508-1 through 508-3 are converted by receiving circuits 507-1 through 507-3 into reception signals, which are processed by reception signal processing circuit 510 into reception data that is output. Reception signal processing circuit 510 also outputs a channel matrix. The MIMO radio communication device is a radio communications device made up of a transmitter with antennas and a receiver with antennas, or a plurality of antennas and a plurality of transceivers, and perform communications according to a spatial multiplexing process.
To the extent of the number of communications paths (independent spatial transmission paths), including multipaths, which can be orthogonalized, the transmission rate is proportional to the number of antennas (either the number of transmission antennas or the number of reception antennas which is smaller). Therefore, the transmission rate can be increased while using the same frequency and the same time. Using time-space coding, a spatial diversity effect is produced to obtain a good SNR (Signal to Noise Ratio).
In general, as the frequency is higher, the radio wave tends to travel straighter, which differentiates a propagative environment. The frequency at which the propagative environment changes is said to be about 10 GHz. Beyond that frequency, it is difficult to perform non-line-of-sight communications. For example, according to“Propagation data and prediction methods for the planning of indoor radio communication systems and radio local area networks in the frequency range 900 MHz to 100 GHz”, ITU-R, P. 1238-3, Apr. 2003, power loss coefficients representing the attenuated amount of radio waves over the distance that the radio waves are propagated are in the range from 28 to 32 in the frequency range from 0.9 to 5.2 GHz in offices, and 22 at the frequency of 60 GHz. Since the power loss coefficient is 20 for a free space loss, scattering and diffraction are considered to be less influential at high frequencies such as 60 GHz. Multipaths are considered to be relatively few though the intensity of radio waves is occasionally strong in some multipaths. A radio system employing millimeter waves, e.g., in the 60 GHz band is described in a document by K. Ohata, et. al. (IEEE MTT-S International Microwave Symposium. Digest, Jun. 2003. pp. 373-376). The modulation process that is used is ASK (digital amplitude modulation), and a high rate of 1.25 Gbits/sec. is achieved for radio communications.